Ethylene has been recognized as a plant hormone since the turn of the century when its effect on pea seedling development was first described. Neljubow (1901), Pflanzen Beih. Bot. Zentralb. 10:128-139. Since then, numerous reports have appeared which demonstrate that ethylene is an endogenous regulator of growth and development in higher plants. For example, ethylene has been implicated in seed dormancy, seedling growth, flower initiation, leaf abscission, senescence and fruit ripening. Ethylene is a plant hormone whose biosynthesis is induced by environmental stress such as oxygen deficiency, wounding, pathogen invasion and flooding.
Recently, genes encoding some of the enzymes involved in ethylene biosynthesis have been cloned. Sato, et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86:6621-6625; Nakajima, et al. (1990) Plant Cell Phys. Physiol. 29:989-996; Van Der Straeten, et al. (1990) Proc. Natl. Acad. Sci U.S.A. 87:4859-4963; Hamilton, et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88:7434-7437; and Spanu, et al. (1991) EMBO J. 10:2007-2013. The pathway for ethylene biosynthesis is shown in FIG. 1. As can be seen the amino acid methionine is converted to S-adenosyl-methionine (SAM) by SAM synthetase which in turn is converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Adams, et al. (1979) Proc. Natl. Acad. Sci. U.S.A. 76:170-174. The ACC is then converted to ethylene by way of the enzyme ACC oxidase. Yang, et al. (1984) Annu. Rev. Plant. Physiol. 35:155-189.
A number of approaches have been taken in an attempt to control ethylene biosynthesis to thereby control fruit ripening. Oeller, et al. (1991) Science 254:437-439 report that expression of an antisense RNA to ACC synthase inhibits fruit ripening in tomato plants. Hamilton, et al. (1990) Nature 346:284-287 report the use of an antisense TOM13 (ACC oxidase) gene in transgenic plants. Picton et al. (1993) Plant Journal 3:469-481, report altered fruit ripening and leaf senesence in tomatoes expressing an antisense ethylene-forming enzyme.
In a second approach, ethylene biosynthesis was reportedly modulated by expressing an ACC deaminase in plant tissue to lower the level of ACC available for conversion to ethylene. See PCT publication No. WO92/12249 published Jul. 23, 1992, and Klee et al. (1991) Plant Cell 3:1187-1193.
While a substantial amount of information has been gathered regarding the biosynthesis of ethylene, very little is known about how ethylene controls plant development. Although several reports indicate that a high affinity binding site for ethylene is present in plant tissues, such receptors have not been identified. Jerie, et al. (1979) Planta 144:503; Sisler (1979) Plant Physiol. 64:538; Sisler, et al. (1990) Plant Growth Reg. 9:157-164, and Sisler (1990) "Ethylene-Binding Component in Plants", The Plant Hormone Ethylene, A. K. Mattoo and J. C. Suttle, eds. (Boston) C.R.C. Press, Inc., pp. 81-90. In Arabidopsis, several categories of mutants have been reported. In the first two categories, mutants were reported which produce excess ethylene or reduced ethylene as compared to the wild-type. Guzman, et al. (1990) The Plant Cell 2:513-523. In a third category, mutants failed to respond to ethylene. Id.; Bleecker, et al. (1988) Science 241:1086-1089, Harpham, et al. (1991) Ann. of Botany 68:55-61. The observed insensitivity to ethylene was described as being either a dominant or recessive mutation. Id.
Based upon the foregoing, it is clear that the genetic basis and molecular mechanism of ethylene interaction with plants has not been clearly delineated. Given the wide range of functions regulated by ethylene and the previous attempts to control ethylene function by regulating its synthesis, it would be desirable to have an alternate approach to modulate growth and development in various plant tissues such as fruits, vegetables and flowers by altering the interaction of ethylene with plant tissue.
Accordingly, it is an object of the invention to provide isolated nucleic acids comprising an ethylene response (ETR) nucleic acid.
In addition, it is an object to provide modifications to such ETR nucleic acids to substitute, insert and/or delete one or more nucleotides so as to substitute, insert and/or delete one or more amino acid residues in the protein encoded by the ETR nucleic acid.
Still further, it is an object to provide plant cells transformed with one or more modified ETR nucleic acids. Such transformed plant cells can be used to produce transformed plants wherein the phenotype vis-a-vis the response of one or more tissues of the plant to ethylene is modulated.